1. Field of the Invention
The present invention concerns a method of removing nitrogen oxides in exhaust gases from a diesel engines.
2. Description of the Prior Art
As a method of removing nitrogen oxides in an oxidative atmosphere, selective reducing denitration method using ammonia as a reducing agent has been considered most effective in an oxidative atmosphere since nitrogen oxides and ammonia are brought into selective reaction free from the effect of oxidation concentration in exhaust gases and, accordingly, it has been applied generally for the purification of exhaust gases from stationary or gas generation sources such as boilers in thermoelectric power plants and heating furnaces.
On the other hand, various examples of applying the ammonia selective reduction method have been disclosed also for the reduction of nitrogen oxides in exhaust gases from internal combustion engines. For instance, there has been disclosed, for example, a method of supplying ammonia in proportion with the amount of fuel consumption into exhaust gases and the resultant gas mixture is passed through a reactor charged with a pellet-like catalyst thereby removing nitrogen oxide (refer to Japanese Patent Publication Sho 58-501001).
However, in a case of diesel engines, since the changes of engine loads are remarkable as compared with those in boilers, and it causes abrupt changed in the amount of the exhaust gases and the concentration of nitrogen oxides, the foregoing prior art can not attain a sufficient effect for precisely controlling the amount of ammonia in accordance with such changes. Accordingly, there has been left a problem in removing nitrogen oxides in exhaust gases at a high efficiency and restricting discharged ammonia as low as possible. Further, efficiency removing of nitrogen oxides in exhaust gases is concerned with a reaction efficiency in a reactor substantially in a proportional relationship. Then, for improving the reaction efficiency, it is important that ammonia supplied to the exhaust gases is mixed uniformly with the nitrogen oxides in the gases.
Then, a mixed method using a buffle plate, venturi or packing has generally been employed. However, a method of supplying ammonia by disposing a plurality of nozzle ports to an exhaust pipe at the upstream to the reactor between a superfeeder and the reactor, and a method and an apparatus for supplying ammonia through a supply port in the form of a spray nozzle should not hinder the operation of the engine itself.
In this regard, the mixing method of using the buffle plate, venturi or the packing material results in the reduction of an engine power in a diesel engine due to great pressure loss, which worsens the fuel cost of the engine and results in a disadvantage to the engine itself.
Further, in the mixing method of disposing a plurality of nozzles in an exhaust gas flow channel at the upstream on the side of the reactor between the super feeder and the reactor for supplying ammonia, it is difficult to uniformly mix ammonia to the exhaust gases, in which the efficiency of removing nitrogen oxides in the exhaust gases on a denitrating catalyst is rather reduced but not improved and, according, this method can not be said to provide a high reaction efficiency.
Further, in the mixing method of supplying ammonia from the spray nozzle, since dusts contained in the exhaust gases cause clogging to the spray port, this may worsen the spraying of ammonia and clog the nozzle to require frequent cleaning or displacement of nozzles.
In addition, in a case of a relatively small sized diesel engine, it is also important that the exhaust gas processing system therefor is inexpensive.